Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactant fluids to power electrical apparatus, such as apparatus on-board space vehicles, transportation vehicles, or as on-site generators for buildings. A plurality of planar fuel cells are typically arranged into a cell stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids as part of a fuel cell power plant. Each individual fuel cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A fuel cell may also include a water transport plate, or a separator plate, as is well known.
The fuel cell stack produces electricity from reducing fluid and process oxidant streams. As shown in the simplified schematic drawing of a prior art fuel cell stack in FIG. 1, the prior art fuel cell stack 10 includes a reaction portion 12 formed from a plurality of fuel cells 14 stacked adjacent each other that produce electricity in a well-known manner. The plurality of fuel cells 14 includes a first end cell 16 and opposed second end cell 18 at opposed ends of the reaction portion 12 of the fuel cell stack 10. First and second pressure plates 20, 22 overlie the end cells 16, 18 and the pressure plates 20, 22 are secured to each other typically by a plurality of tie rod nut assemblies (not shown) to apply a compressive load to the stack to seal a plurality of compression seals (not shown) within the stack 10. Most known pressure plates 20, 22 are made of large, conductive metal materials.
During operation of such fuel cell stacks 10, creation of heat by the stack 10, and flow of compressed fluids through the stack 10 results in expansion and contraction of dimensions of the stack 10 within operating dynamic limits of the fuel cell stack 10. Therefore, to permit expansion of the stack 10 within such limits, known fuel cell stacks 10 utilize a system to maintain load follow-up. Load follow-up is the distance at which a pressure plate continues loading while the fuel cell stack creeps. In a design with no load follow-up, for example, if an initial load was 60 pounds per square inch (“PSI”), and the stack were to creep, the load would fall off or lessen. For example, if the fuel cell stack creeped 0.050 inches the load would be 50 PSI. A load follow-up distance is the distance the stack can creep while the load remains about constant. (“About constant” is to mean that a slope of load decrease as a function of stack creep is greatly reduced).
A common load follow-up system includes one or more belleville washers (not shown) secured to each tie rod of the stack 10, between a tie rod securing nut (not shown) and the pressure plates 20, 22. Such a load follow-up system provides for limited expansion within the operating dynamic limits of the stack 10 while applying a constant minimum load to the stack 10. Traditionally, to achieve an effective load follow-up system, known fuel cell stacks 10 have utilized large, heavy, metallic pressure plates 20, 22 as part of the load follow-up system.
Such known fuel cell stacks 10 have given rise to many problems related to a high thermal capacity of the large pressure plates 20, 22. For example, during a “bootstrap” start up from subfreezing conditions, preferably no auxiliary heated fluids are applied to the fuel cell stack 10, while a reducing fluid, such as hydrogen, and an oxidant, such as oxygen or air, are supplied to the fuel cells 14. In a fuel cell 14 utilizing a proton exchange membrane (“PEM”) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of an anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy. Electricity produced by the fuel cells 14 flows into and/or through the conductive pressure plates 20, 22.
During such a “bootstrap” start up, the fuel cells 14 that are in a central region of the stack 10 quickly rise in temperature compared to the end cells 16, 18 that are adjacent opposed ends of the stack 10. The end cells 16, 18 heat up more slowly because heat generated by the end cells 16, 18 is rapidly conducted into the large, conductive metallic pressure plates 20, 22. If a temperature of the end cells 16, 18 is not quickly raised to greater than 0 degrees Celsius (“C”), water in water transport plates within the stack 10 will remain frozen thereby preventing removal of product water, which results in the end cells 16, 18 being flooded with fuel cell product water. The flooding of the end cells 16, 18 retards reactant fluids from reaching catalysts of the end cells 16, 18 and may result in a negative voltage in the end cells 16, 18. The negative voltage in the end cells 16, 18 may result in hydrogen gas evolution at cathode electrodes and/or corrosion of carbon support layers of electrodes of the cells 16, 18. Such occurrences would degrade the performance and long-term stability of the fuel cell stack 10.
Many efforts have been undertaken to resolve such problems. For example, U.S. Pat. No. 6,764,786 that issued on Jul. 20, 2004, to Morrow et al. discloses a pressure plate that is made of an electrically non-conductive, non-metallic, fiber reinforced composite material, so that the pressure plate is light, compact and has a low thermal capacity. Similarly U.S. Pat. No. 6,824,901 that issued on Nov. 30, 2004, to Raiser at al. discloses a fuel cell stack having thermal insulting spacers between pressure plates and end cells. More recently, U.S. Pat. No. 8,354,197 issued on Jan. 15, 2013 to Lake et al. and discloses an integrated end plate assembly using a current collector and an electrically non-conductive pressure plate. A backbone nests within a central backbone support plane and peripheral deflection planes, and the “dog bone” shaped backbone includes tie-rod ends with throughbores that permit deflection of the tie-rod ends within the deflection planes to permit limited expansion and contraction of the fuel cell stack while maintaining a predetermined load follow-up. Unfortunately, while this design provides limited expansion and contraction of the fuel cells of the stack, to achieve an even application of the load follow-up, the pressure plate must be quite thick to achieve a necessary rigidity to apply an even application of the load to the fuel cells. This substantially limits the efficiency of the Lake et al. design.
All of these Patents are owned by the assignee of all rights in the present disclosure. While known fuel cell stacks have limited such problems related to the high thermal capacity of large, metallic pressure plates, such fuel cell stacks still present substantial challenges for efficient operation, especially for PEN electrolyte based fuel cells within fuel cell stacks that undergo frequent start-stop cycles in varying ambient conditions, such as in powering transportation vehicles.
Accordingly, there is a need for a fuel cell stack having end cells wherein temperatures of the end cells can be raised to greater than 0° C. as quickly as possible during start up from subfreezing conditions, and that can also provide an efficient load follow-up system.